Spark arrester

ABSTRACT

The triggering electrode is butted against an opening in the cathode, against the inside face of this cathode, closing a solid dielectric sheet with an opening perforated in it. This permits, on the one hand, the precise positioning of this electrode with respect to the cathode, with a small interval, 12 microns, for example, and on the other hand, the lowering of the voltage necessary to provide the pulse for the triggering. The invention applies particularly to rapid spark arresters where the triggering voltage is less than 2 kV.

I Umted States Patent [151 3,702,41 1 Destree 1 Nov. 7, 1972 [54] SPARK ARRESTER [56] References Cited [72] Inventor: Rene Destree, Viry-Chatillon, UNITED STATES PATENTS France 3,465,192 9/1969 Lafferty ..3l3/l98 [73] Assignee: Compagnie Generale DElectricite,

Paris, France Primary Examiner-J0hn Kominski 14, Attorney-Sughrue, ROthWell, MiOn, Zinn & Macpeak [21] Appl. No.: 180 298 [57] ABSTRACT The triggering electrode is butted against an opening [30] Foreign Application Priority Data in the cathode, against the inside face of this cathode.

closing a solid dielectric sheet with an opening per- Sept. 16, 1970 France ..7033577 forated in it This permits, on h one h d h precise positioning of this electrode with respect to US. Cl. the cathode with a mall interval microns for ex. 3 13/356 ample, and on the other hand, the lowering of the [5 1] Int. Cl .1101] 17/30 voltage necessary to provide the pulse for the trigger- [58] Field of Search ..3l3/356, 208, 197, 198 ing, The invention applies particularly to rapid spark arresters where the triggering voltage is less than 2 kV.

7 Claims, 2 Drawing Figures PATENT'E'Dnnv 1 I972 SHEET 1 OF 2 FIG.1

SPARK ARRESTER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to spark arresters triggered in a gas and more particularly to those spark arresters which permit the discharge of a large energy joules, for example) in a very short time which can be counted in nanoseconds.

2. Description of the Prior Art Many triggered spark arresters are known involving, in addition to the principal electrodes (cathode and anode) between which the principal discharge takes place, a third electrode which hereafter is called the triggering electrode and which is arranged in the cathode, the entire assembly presenting a coaxial structure.

The term spark arrester in a gas is used in describing a spark arrester between the principal electrodes of which there is a gas pressure that is sufficiently great so that the majority of the ions, which transport the principal discharge, will come from molecules of the ambient gas and that only a minority of these ions comes from electrodes. The pressure in such a spark arrester must be greater than 1/100 of normal atmospheric pressure.

A direct continuous voltage volts, for example) can be applied between the cathode and the anode of a spark arrester triggered in a gas, for example, by means of the connection of this cathode and this anode to the terminals of a condenser charged by this voltage. When this voltage is smaller than the breakdown voltage between the cathode and the anode and when it is greater than a threshold, which is a function, like the breakdown voltage, of the geometrical arrangement of the spark arrester and of the nature of the pressure of the gas washing the electrodes, and when a suitable electrical impulse is applied to the triggering electrode, so as to create a potential difference, for example, amounting to several KV, between this triggering electrode and the cathode, the electrical breakdown occurs between this triggering electrode and the cathode, and this breakdown starts the discharge between the principal electrodes.

The usefulness of the spark arrester is due to the fact that the energy, which must be applied to the triggering electrode, can be very much smaller than the controlled energy.

In these spark arresters, the cathode involves an axial opening, in which is placed the active end of the triggering electrode, the body of the latter being inside the body of the cathode. The active parts of the cathode and the triggering anode lie approximately in one and the same plane so that, on the one hand, the triggering discharge can disturb the electrical field between the principal electrode and, on the other hand, the presence of the triggering electrode will not in any major fashion intervene in the principal discharge.

It is often important to be able to trigger the spark arrester by means of an electrical impulse presenting not only a small energy but also a small potential difference, less than 2 kilovolts. Various solutions have been proposed for this purpose:

One consists in reducing the distance separating the triggering electrode from the opposite parts of the cathode, it being understood here that, from this viewpoint, there is an optimum distance which depends on the nature and the pressure of the ambient gas, and below which it is useless and harmful to descend. In the case of atmospheric-pressure spark arresters, this distance is on the order of about 10 microns. Great triggering precision, in terms of time, requires this distance to be made with precision. Now, considering the insulation which must be provided between the triggering electrode and the cathode, the creation and maintenance of a distance as small as this pose difficult design problems.

For the same purpose, it is also known that one can occupy the space between the triggering electrode and the cathode with a solid dielectric. The triggering discharge then takes place on the surface of this dielectric. In effect, an electrical discharge preferably, in a dielectric environment follows the interfaces between dielectrics of different kinds, and, more precisely, in the arrangement described, the interface between the solid dielectric and the gaseous dielectric. This boils down to saying that the voltage necessary for breakdown is diminished.

Nevertheless, one wants this voltage to be very small, and it remains necessary to give the solid dielectric, separating the triggering electrode from the cathode, a small thickness. Now, the arrangement adopted in the known spark arresters is such that if one looks at the active face of the cathode, one sees, on the axis, a circle delimiting the active end of the triggering electrode, around this circle is a circular crown of dielectric solid, this crown being itself surrounded by the active portion of the cathode. The solid dielectric thus presents the form of a cylindrical sleeve surrounding the triggering electrode and coaxial to the latter.

It is necessary for the geometrical arrangements adopted here to be preserved in the course of time, that is to say, it is necessary to make sure that the solid dielectric will not have its surface progressively disturbed by the discharges which follow each other. This is why, in the known spark arresters, this dielectric is made in the form of a ceramic, for example, of fritted alumina. Now, it is practically impossible to give such materials the form of a cylindrical sleeve with very small thickness. This is why the known spark arresters of this type involve a ceramic sleeve with a relatively great thickness (more than 50 microns), which has been suitably treated so as to promote the development of an electrical discharge on the surface, that is to say, to reduce the voltage necessary for triggering. These various arrangements therefore make it possible to lower the breakdown voltage around 2 kv only by means of a considerable increase in the cost of producing the spark arrester.

Furthermore, known spark arresters are triggered in a vacuum, in connection with which is encountered problems difierent from those that have been outlined. These spark arresters assuming equal size can tolerate higher electrical voltages than spark arresters in a gas but in particular they entail the inconvenience that the discharges produce much in the way of metallic vapors which make the dielectric surfaces conducting, especially the surface of the dielectric that insulates the triggering electrode. In order to reduce this metallization, it has been proposed to use a ring-shaped triggering electrode, arranged coaxially on the edge of the opening made in the cathode, with a ring-shaped dielectric layer separating this triggering electrode from the inside face of this cathode, the diameter of the interior circle of this triggering cathode and of this layer being equal to that of this opening.

The usefulsurface of this dielectric layer, that is to say, its section along the interior circle, is then recessed, or pocketed, with respect to the principal discharge, which diminishes the rapidity of this metallization. This problem of metallization obviously makes it necessary to givethis dielectric surface a sufficient width, that is to say, a sufi'icient layer thickness. This thickness is generally more than 1 millimeter.

SUMMARY OF THE INVENTION One purpose of this invention is to make a spark arrester triggered in a gas, requiring low voltage for its triggering while still involving only low production costs. v a

This invention in particular has the purpose of providing a spark arrester triggered in a gas, involving a first and a second principal electrode, arranged on either side of an interval filled with gas, and presenting opposing active faces between which a principal electrical discharge may be triggered. Said first electrode is hollowed out so as to form an interior space leading into an opening in its active face, a triggering electrode being arranged in said interior space and involving an active end which closes off said opening at least partially. A solid dielectric occupies the space between said active end of the triggering electrode and the edge of said opening, characterized by the fact that said triggering electrode, at its active end, has a larger cross section than said opening, so that a peripheral zone of the terminal face of this active end is masked by the edge of said opening, the material of said first elec- 1 trodebein g included along the edge of said opening between an external face, which is opposite said second electrode and which at least partially constitutes said active face, and an inside face, which is opposite said peripheral zone. Said solid dielectric has the form of a thin layer which occupies the space between said. peripheral zone and said inside face and which presents an opening that is essentially coextensive with said opening of the first electrode, the thickness of this layer being smaller than 50 microns.

With the help of the attached FIGS. 1 and 2, it is now described below, by way of illustration and without any restrictions, two ways of implementing this invention. The element that correspond to each other in these two figures have been given the same reference numbers.

BRIEF DESCRIPTION OF THE DRAWINGS distance between the active faces 6 and 8 of cathode 2 and anode 4 is equal to 2 mm. Under these conditions, a voltage of 9 kv can be maintained permanently between these electrodes, without any breakdown.

Cathode 2 is hollow and is perforated with-an opening 10, the entireassemblypresenting revolution symmetry. On the'axis, there is arranged a triggering electrode 12, having the form of a solid cylinder whose upper face 14 is opposite opening 10. The diameter of the triggering electrode 12 is larger than that of opening 10, so that a peripheral zone 16 of this upper face is masked by the edge .of opening 10. On either side of this opening, cathode 2 has the form of a thin wall with a thickness of 0.3 mm, one of the faces of this wall being the active face of cathode 2, the other one constituting the above mentioned inside face. Against this inside face is arranged a flat crown 18, cut out of a sheet of terephthalate of polyethylene'with a thickness of 12 microns. i

The position of the triggering electrode 12 is such that this circular crown will be closed between the peripheral zone 16 and the inside face of cathode 2. The central opening of the dielectric crown 18 presents the same diameter as the opening 10 and is arranged opposite this opening 10.

The voltage, controlled by the spark arrester, is applied between electrodes 2- and 4 and the principal electrical discharge takes place between the active faces of these two electrodes. This discharge is brought about by a triggering pulse applied between the triggering electrode 12 and the cathode 2. The spark arrester according to the invention makes it possible to control an energy of 10 Joules with a triggering pulse having an energy less than l m] and a voltage less than I kv.

The remarkable results obtained according to the invention are connected with the following facts:

First of all, the distance between face 14 of the triggering electrode 12 and the inside face of cathode 2 is FIG. 1 shows an axial cross section view of a first version of the invention.

FIG. 2 shows an axial cross section view of a second version of the invention.

DESCRIPTION OF THE PREFERRED I EMBODIMENTS very small (12 microns, in the example selected). It could be lowered,-for example, to 6 microns, without any special difficulty, because the positioning of electrode 12 can be guaranteed with precision by putting this electrode so that it will abut against the inside face of cathode'2, by means of crown 18. It is easy and inexpensive to arrange dielectric sheets with the desired thickness. I

The performance is retained after numerous discharges because the energy necessary for triggering is low, which prevents the deterioration of the dielectric. In effect, the triggering discharge is propagated on the interface between the solid dielectric and the gaseous dielectric, that is to say, here, in the direction in terms of the thickness of the crown 18, on the inside edge of this crown. If a very large energy were dissipated in each triggering pulse, these inside edges would deteriorate rapidly and we would in practice have to use a ceramic dielectric.

The dielectric crown 18, cut out of a thin sheet, according to the invention, performs two functions: a mechanical positioning functioning, making it possible in a simple and precise manner to define a distance which can be very much smaller than 50 microns; and an electrical function not only of insulation but also of lowering the breakdown voltage because of the preferential routing of the triggering discharge to the edge of the opening made in this crown.

FIG. 2 has the same elements as in FIG. 1 but the peripheral zone 16 has the form of a conical surface open toward the triggering electrode 12 and coaxial to the assembly of the spark arrester. This peripheral zone is coated with a layer 18 of metallic oxide which has been deposited on this triggering electrode. This oxide layer is 12 microns thick.

All around opening 10, cathode 2, when cut by a meridian plane, presents the form of a wedge with a truncated point, the zone of truncation constituting the edge of opening 10. This edge has a width of 3 millimeters, essentially parallel to the axis of revolution. The metallic oxide constituting layer 18 can be alumina, for example.

However, one can make the triggering electrode 12 of an oxidable metal, such as titanium and one can create the metallic oxide layer 18 by means of oxidation of this metal. One can also make the layer 18, not on the peripheral zone of the triggering electrode 12, but on the inside face of cathode 2. As before, the term inside face of cathode 2 refers to the face of this electrode which isopposite the peripheral zone 16. In the vicinity of the edge of opening 10, the material constituting cathode 2 thus appears to be limited, on the one hand, between an external face, which constitutes a portion of the active face of this electrode, and, on the other hand, this internal face. Like the peripheral zone of the triggering electrode 12, this inside face has the form of a trunk with a conical revolution surface open toward this triggering electrode, with the same angle of opening, so that the space between this inside face and this peripheral zone will be entirely occupied by oxide layer 18. In the production of the spark arrester, this kind of arrangement mades is possible to have layer 18 play the role of a thickness wedge. To accomplish this, it suffices for the triggering electrode 12 to be pushed upward until it comes to butt against the inside face of cathode 2. This arrangement furthermore makes it possible to ensure a certain centering of the triggering electrode 12 with respect to cathode 2.

On the other hand, the terminal face 14 of the triggering electrode 12 is hollow, over its entire surface inside said peripheral zone, so as to form a basin. This arrangement permits a penetration of the principal electrical field to the interior of opening 10. Here the term principal field refers to the field established between cathode 2 and anode 4. This arrangement facilitates the triggering of the principal discharge when an auxiliary discharge is triggered between the triggering electrode 12 and the edge of opening of cathode 2.

The edge of opening 10, the edge of the opening made in layer 18 opposite opening 10, and the lateral walls of the basin made in the triggering electrode 12 constitute three adjacent zones of a conical revolution surface open toward anode 4. This arrangement permits an auxiliary discharge sparking between the lateral walls of the hollow basin made in the terminal face 14 and the edge of opening 10 of cathode 2 to reach a zone where the principal electrical field mentioned earlier is more intense. It is, however, quite evident that this result is obtained as a function of the form of only a portion of the walls of the basin in the terminal face 14, said portion being the one that is adjacent to the edge of this basin.

It is quite evident that certain arrangements, which have been described here, can be used independently of each other while still offering advantages. The constitution of the dielectric layer 18 by a metallic oxide makes it easily possible to give it a small thickness, suitable for lowering the voltage required for triggering. Making the hollow basin in the terminal face 14 facilitates the penetration of the principal field even if the lateral walls of this basin are not conical but cylindrical, that is to say, parallel to the axis of the spark arrester.

Finally, a last arrangement which can be used here independently is the conical form of the inside face of cathode 2, of layer 18, and of the peripheral zone of the triggering electrode 12.

What is claimed is:

1. In a spark arrester triggered in a gas, involving a first and a second principal electrode, arranged on either side of an interval filled with gas, and presenting opposing active faces between which a principal electrical discharge may be triggered, said first electrode being hollow so'as to form an inside space leading to an opening in its active face, a triggering electrode being arranged in said inside space and involving an active end closing off said opening at least partially, a solid dielectric occupying the space between said active end of the triggering electrode and the edge of said opening, the improvement wherein: said triggering electrode, at its active end, presents a larger cross section than said opening, so that a peripheral zone of the terminal face of this active end is masked by the edge of said opening, the material of said first electrode being included, at the edge of said opening, between an outside face which is opposite said second electrode and constitutes at least partially said active face and an inside face which is opposite said peripheral zone, said solid dielectric having the shape of a thin layer which occupies the space between said peripheral zone and said inside face and which presents an opening that is essentially co-extensive with said opening of the first electrode, and the thickness of said layer being less than 50 microns.

2. The spark arrester according to claim 1, wherein: said thin dielectric layer is a sheet of organic polymer.

3. The spark arrester according to claim 1, wherein: said thin dielectric layer is a layer of metallic oxide formed on one of two surfaces, where one surface is constituted by said peripheral zone and the other one is constituted by said inside face.

4. The spark arrester according to claim 3, wherein: said metallic oxide is an oxide of the metal constituting that surface on which said thin layer is formed.

5. The spark arrester according to claim 1, wherein: the terminal face of the triggering electrode is hollow over its entire surface inside said peripheral zone to form a basin.

6. The spark arrester according to claim 5, wherein: the edge of said opening of the first electrode, the edge of said opening of said thin dielectric layer, and at least a portion of the walls of said basin in the triggering electrode that is adjacent to the edge thereof, constitute three adjacent zones of an essentially conical surface opening toward said second principal electrode.

7. The spark arrester according to claim 1, wherein: said inside face of the first electrode, said thin dielectric layer, and said peripheral zone of the triggering electrode form essentially conical surfaces that open toward said triggering electrode. 

1. In a spark arrester triggered in a gas, involving a first and a second principal electrode, arranged on either side of an interval filled with gas, and presenting opposing ''''active faces'''' between which a principal electrical discharge may be triggered, said first electrode being hollow so as to form an ''''inside space'''' leading to an opening in its active face, a triggering electrode being arranged in said inside space and involving an ''''active end'''' closing off said opening at least partially, a solid dielectric occupying the space between said active end of the triggering electrode and the edge of said opening, the improvement wherein: said triggering electrode, at its active end, presents a larger cross section than said opening, so that a peripheral zone of the terminal face of this active end is masked by the edge of said opening, the material of said first electrode being included, at the edge of said opening, between an outside face which is opposite said second electrode and constitutes at least partially said active face and an inside face which is opposite said peripheral zone, said solid dielectric having the shape of a thin layer which occupies the space between said peripheral zone and said inside face and which presents an opening that is essentially co-extensive with said opening of the first electrode, and the thickness of said layer being less than 50 microns.
 2. The spark arrester according to claim 1, wherein: said thin dielectric layer is a sheet of organic polymer.
 3. The spark arrester according to claim 1, wherein: said thin dielectric layer is a layer of metallic oxide formed on one of two surfaces, where one surface is constituted by said peripheral zone and the other one is constituted by said inside face.
 4. The spark arrester according to claim 3, wherein: said metallic oxide is an oxide of the metal constituting that surface on which said thin layer is formed.
 5. The spark arrester according to claim 1, wherein: the terminal face of the triggering electrode is hollow over its entire surface inside said peripheral zone to form a basin.
 6. The spark arrester according to claim 5, wherein: the edge of said opening of the first electrode, the edge of said opening of said thin dielectric layer, and at least a portion of the walls of said basin in the triggering electrode that is adjacent to the edge thereof, constitute three adjacent zones of an essentially conical surface opening toward said second principal electrode.
 7. The spark arrester according to claim 1, wherein: said inside face of the first electrode, said thin dielectric layer, and said peripheral zone of the triggering electrode form essentially conical surfaces that open toward said triggering electrode. 